Liquid crystal displays (LCDs) are used in a wide variety of computers and consumer devices such as TVs. A back-lit LCD is an array of pixels in which each pixel acts as a shutter that either passes or blocks light from a light source that is located behind the pixel. Color displays are implemented by equipping the pixels with color filters such that each pixel transmits or blocks light of a particular color. The intensity of the light from each pixel is set by the time the pixel is in the transmissive state.
The display is typically illuminated by a white light source that provides a uniform intensity of light across the back surface of the display. Illumination sources based on fluorescent lights are particularly attractive because of their high light output per watt-hour of power consumed. However, such sources require high driving voltages which makes them less attractive for battery operated devices.
As a result, there has been considerable interest in utilizing light sources based on LEDs in such applications. LEDs have similar electrical efficiency and long lifetimes. In addition, the driving voltages needed are compatible with the battery power available on most portable devices.
The illumination system typically utilizes some form of light box or light pipe behind the LCD array. Light is injected into this light box at the periphery of the light box. The surface of the light box opposite to the surface that is adjacent to the LCD array has some form of scattering covering that scatters the light so that the back surface of the LCD is uniformly illuminated.
The thickness of the light source is limited by the thickness of the light box. The thickness of the display is particularly important in displays used for laptop computers and handheld devices such as PDAs and cellular telephones, as the display thickness limits the overall thickness of the device. Some of these portable devices require light boxes that are less than 10 mm thick. As the thickness of the light box is reduced, the efficient injection of light into the light box from the sides in a manner that assures uniform illumination becomes more difficult.
In addition, large displays present a power dissipation problem. Most of the power applied to the light source is converted to heat, rather than light. Since LED-based light sources are confined to the periphery of the light box, this heat is concentrated in the region around the periphery of the light box. Transferring the heat from the edge of the light pipe to a surface that is better adapted for transferring the heat to the environment outside the display poses significant problems.